Practical wire corrosion concerns

[rawelk]

On old motor installations it isn't uncommon to find green corrosion in copper wires. By the time this occurs there are often other reasons such as insulation degradation to support replacing the wire runs, but regardless, I've always believed this corrosion sufficient to recommend replacing the wire, or, if possible, cutting it back to 'clean' wire, and re-terminating.

In this particular case I have two runs of 500MCM per terminal on a 400HP/500V DC motor which has been in service since approximately 1980. The motor is used to turn an extruder screw which processes polystyrene (but not PVC, so chlorides shouldn't be an issue). Two 350MCM tinned copper leads per pole come from the motor, and are terminated with solderless crimp-on tinned copper ring terminals. These are bolted to aluminum two wire compression 'bugs' for the 500MCM pair.

I've attached a photo of the wire under consideration.

My questions
1). I've always assumed this corrosion is copper carbonate. True? False? Or could it be a combination of carbonate, and other copper compounds? I'm guessing the blacker stuff is cupric oxide (heating copper carbonate creates it and carbon dioxide).

2). I was looking around the web for a fact-based citation on why green corroded wire should be replaced (i.e. - bulk resistivity of the corrosion layer is such-and-such higher than copper, and causes overheating, etc.) but, while I found many opinions supporting the replacement idea, didn't find anything authoritative.

Does anybody know of a good write-up?

 

[HCBFlash]

The Electrical Power Engineering forum might be more helpful / appropriate to get this info.

My own experience with this corrosion almost always involves moisture as well as heat. Repair / replacement of wiring would be primarily evaluated by the damage to the conductors insulation.

Outdoor overhead copper wires always discolor similar to yours quite quickly.


flash

 

[electricpete]

Is the corrosion on the motor T-leads or the power cables?

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[rawelk]

This corrosion is on the power cables from the DC drive enclosure to the motor conduit box.

 

[gepman]

You may want to try the Copper Development Association. There is an article on corrosion and copper connectors at

http://www.copper.org/applications/electrical/building/homepage.html

You could also send them this question. Although you may think that they have an interest in replacing corroded copper they also have an interest to state that it holds up well to slightly corrosive conditions.

You are correct that the green is copper carbonate. I only use tinned copper for marine applications but in standard building or industrial applications I almost exclusively use untinned copper products.